TSC simulation of ITER plasma termination scenario with stable H-L mode transition and avoidance of radiation collapse

Nakamura, Yukiharu*; Miyamoto, Seiji; Sugie, Tatsuo; Kusama, Yoshinori; Yoshino, Ryuji

The ITER termination scenario from 15 MA to 1.5 MA (500 s t 700 s) [A.A. Kavin , Progress report of plasma startup and termination, ITER_D_2F55U5 (2008)] was reviewed by self-consistent simulations with the TSC code, comprised of newly developed D-T fuelling and pumping-out system. The dynamics of a forced H- to L-mode transition, reduction in the plasma density while building-up of the in-vessel neutral gas, disappearance of the edge BS current and consequent jump in the internal inductance, were investigated to assess the performance of the ITER pump-out system. It was newly demonstrated that the forced H-L mode transition may trigger a radiation collapse, consequently terminating the discharge even in magnetic divertor configuration, if the effective pumping speed is insufficient or if the wall retention of the neutral particle remains saturated. In order to avoid the radiation collapse during plasma termination, off-axis heating and current drive with 170 GHz O-mode EC wave was utilized and shown to be effective.